During repair or redesign of valves, especially large valves in nuclear power plants, a normal method may comprise machining of the old valve seat, welding of new seat material and machining of the welded-on material into the desired geometry for the new valve seat. This can be done either on a cut-off valve in a workshop, or in situ in the pipeline. This entails extensive work. The present invention relates to a device by means of which this work is considerably simplified.
The device comprises a prefabricated annular first valve seat provided with a supporting ring with an outer surface. The first valve seat is intended to replace a second valve seat, removed from the valve to be repaired, by fitting the supporting ring into a recess at the place of the second valve seat in the valve. According to the invention, the supporting ring is combined with a ring of memory metal which is adapted, with the supporting ring in position in the recess, upon heating above the transition temperature of the memory metal with ensuing deformation of the ring, to fix the supporting ring in the recess by expansion either in a space around the supporting ring between its outer surface and the inner surface of the recess, or inside the supporting ring towards the inner surface thereof such that at least part of the outer surface of the supporting ring is sealingly pressed against the inner surface of the recess.
The device permits work with repair or redesign of a valve without extensive welding operations carried out in situ in the pipeline in which the valve is positioned. The old second valve seat is removed in a suitable way and a recess for the new first valve seat is machined into the desired shape. The new valve seat combined with a ring of memory metal is placed in the recess and the ring of memory metal is heated in a suitable way above the transition temperature of the memory metal. The ring of memory metal shall then, below the transition temperature, have been deformed into such a shape that the combined device of the first valve seat and the ring of memory metal can be inserted without difficulty into the recess. Above the transition temperature, however, the ring shall have such a shape that it fixes the valve seat into the recess in a reliable manner. This can be done, for example, by the ring being deformed into a smaller diameter below the transition temperature than it has above the transition temperature. It should perhaps be noted that a memory metal is an alloy which may occur in a stable austenitic phase at a temperature above a certain so-called transition temperature and a stable martensitic phase below that temperature. A memory metal, which in the austenitic phase has been given a certain first shape and is then cooled down below the transition temperature to martensitic phase and thereafter deformed into a second shape, has the ability, after heating above the transition temperature to austenitic phase, to resume its first shape, that is, to remember that shape. Certain memory metals have the ability to change their shapes each time the transition temperature is passed. They have a so-called two-way memory effect. In this case, however, it is desirable for the memory metal to be of a so-called one-way type.
This means that the above-mentioned ring of memory metal is given the desired final shape in austenitic phase. Thereafter, it is cooled down below the transition temperature to martensitic phase and deformed into a shape suitable for the mounting of the device in position in the valve. After heating above the transition temperature to austenitic phase, it assumes or attempts to assume the original shape and then maintains this shape even if the temperature should again drop below the transition temperature.